This invention relates to a method for the manufacture of .alpha.-type silicon nitride, and more particularly to a method for the manufacture of finely divided .alpha.-type silicon nitride suitable for use as a raw material for the production of high-strength sintered articles of .alpha.-type silicon nitride.
Silicon nitride has excellent thermal resistance and high-temperature strength. Silicon nitride in a sintered form, therefore, is expected to find utility in various high-temperature structural members such as high-temperature gas turbines, for example. The thermal and mechanical properties of the sintered articles of silicon nitride depend heavily on the attributes of silicon nitride, the raw material for the sintered articles. To achieve the optimum effect, silicon nitride is desired to have as high an .alpha.-type content and possess as small a particle diameter as possible. Among the methods developed and adopted to date for the synthesis of silicon nitride, those widely known in the art are:
(i) nitridation of metallic silicon, PA0 (ii) reaction of silicon tetrachloride and ammonia,
(iii) reductive nitridation of silica (SiO.sub.2) powder by heating the silica powder with carbon powder in a nitrogen atmosphere.
Of these methods, the method (iii) enjoys a number of commercial advantages such as low cost of raw materials used, simplicity and ease of reactions and attendant operations involved in the synthesis, and freedom from use of raw materials capable of corroding the equipment in use.
In the method (iii), however, the nitriding of silica powder does not easily proceed and, in particular, the nitriding of crystalline silica proceeds with great difficulty. Thus, this method is generally carried out using amorphous silica powder. Even when amorphous silica powder of relatively high reactivity is adopted, this method has necessitated adoption of special measures for further enhancing the powder's reactivity. For example, the method of Japanese Patent Publication No. 12320/1976 relies on use of amorphous silica powder having a fluoride or silicofluoride adhered thereto. This method is not very advantageous, however, because addition of a fluoride, etc. results in a fall in the purity of the silicon nitride product.
Further, the product of the method (iii) is generally a mixture of .alpha.-type silicon nitride, .beta.-type silicon nitride, silicon carbide, etc. and occasionally including unaltered silica and silicon oxy-nitride. Thus, the product is apt to contain only a small proportion of the desired .alpha.-type silicon nitride.
A number of methods have been proposed to date with a view to overcoming these drawbacks and obtaining products of a high .alpha.-type silicon nitride content by the method (iii) which promises many commercial advantages. For example, the method of U.S. Pat. No. 4,117,095 contemplates addition of metallic silicon, that of Japanese Patent Publication No. 23917/1979 addition of silicon nitride, silicon carbide, etc. and that of Japanese Unexamined Patent Publication No. 126696/1979 addition of silicon nitrogen imide respectively as third components to the reaction system of the method (iii), and the method of U.S. Pat. No. 4,122,152 involves use of special carbon powder. The methods which resort to addition of third components are not desirable approaches from the economic point of view because the third components specified for addition are more expensive than the main raw materials silica and carbon. The undesirability of these approaches is aggravated by the special requirements imposed on the attributes of such third components themselves such as, for example, the requirement in the method of U.S. Pat. No. 4,117,095 for use of metallic silicon having an average particle diameter of not more than 10.mu.. The method resorting to use of a special carbon powder has a disadvantage that the individual particles of the silicon nitride powder product lack uniformity of shape and have larger diameters than are expected.
In addition to the aforementioned methods available for the synthesis of silicon nitride, the method disclosed by Japanese Unexamined Patent Publication No. 138898/1979 has found wide recognition in the art. This method comprises preparing a mixture of a silicon compound represented by the formula: EQU SiR.sup.1 (R.sup.2).sub.3
(wherein R.sup.1 is an alkyl group having 1 to 4 carbon atoms and R.sup.2 is a hydrogen atom or a chloride atom), a silicon compound represented by the formula: EQU Si(R.sup.3).sub.4
(wherein R.sup.3 is a hydrogen atom, a chlorine atom, or an alkoxy group having 1 to 4 carbon atoms), and carbon powder. Adjusting the mixture to a pH value of not more than 12, produces a precipitate of carbon particles having adsorbed on their surface a silicon compound represented by the formula: EQU (R.sup.4).sub.2x.sbsb.2 Si.sub.2 O.sub.4-x.sbsb.2.yH.sub.2 O
(wherein R.sup.4 is an alkyl group having 1 to 4 carbon atoms, x.sub.2 is an integer having a value of not more than 3, and y denotes the amount of water of adsorption). This precipitate is then nitrided. This method has a disadvantage that it requires use of expensive raw materials and necessitates thermal treatments in two separate stages.
Japanese Unexamined Patent Publication No. 25500/1976 teaches a method which uses, as the raw material for nitriding, a product obtained by impregnating an organic polymer with a solution containing silicon and thermally decomposing the impregnated organic polymer into ashes. The specification of this patent application offers no specific description concerning "the solution containing silicon" and discloses no specific technical measures. Accordingly the disclosure of this specification seems to be entirely speculative.